In U.S. Pat. No. 3,920,043 and U.S. Pat. No. 3,893,483 there is provided a noise-damping sieve assembly arranged at a specific distance from the bottom of the surface duct or recess. The sieve, in one embodiment, is held between two annular portions abutting the side walls of the surface duct, one annular portion of which is used as a range spacer for determining the distance between the sieve and the bottom of the surface duct, while the other annular portion is used to secure the sieve. Besides the actual sieve, two annular portions are also required for the positioning and securing of the sieve in the surface duct.
Sieves are also known formed from one piece and shaped like a kind of basket, wherein the open end of the sieve basket is supported on the bottom of the surface duct, see U.S. Pat. No. 4,157,099. Thus the side wall of the sieve basket is also used to maintain spacing. The securing of the sieve, however, is effected via a separate annular portion.
Embodiments are also known with which a flat sieve is held in the surface duct between an annular portion of this type and a rotating shoulder.
It has been seen that the use of separate annular portions is relatively costly, is more difficult to maintain specific manufacturing tolerances with control discs made of ceramic material.